The present invention relates to a fixing device provided with a heating roller, a fixing roller, and an endless belt stretched between the heating roller and the fixing roller and configured to transfer heat generated from the heating roller to the fixing roller through the endless belt and also relates to an image forming apparatus provided with such a fixing device.
An electrophotographic image forming apparatus is provided with a fixing device that fixes a toner image to a sheet of paper by heating and pressurizing the sheet bearing the toner image on the sheet of paper. Some fixing devices are provided with a heating roller, a fixing roller, and an endless belt stretched between the heating roller and the fixing roller and configured to heat the fixing roller by transferring heat generated from the heating roller to the fixing roller through the endless belt.
In such a fixing device, in order to adjust the tension of the endless belt, for example, the heating roller is biased by a plurality of biasing members in a direction in which the tension of the endless belt increases. In the fixing device, however, since the contact area between the heating roller and the endless belt is large, the meandering in which the endless belt deviates toward one side or deviates toward the other side in the width direction of the endless belt has been liable to occur due to the variation of the biasing force of the biasing member in the axial direction of the heating roller or the fixing roller, in other words, in the width direction of the endless belt.
Thus, instead of biasing the heating roller, a fixing device that is further provided with a tension roller that is brought into pressure contact with the inner peripheral surface of an endless belt has been proposed. In the fixing device, since the tension roller is brought into pressure contact with the endless belt between the heating roller and the fixing roller in a flatter state as compared with the endless belt in contact with the peripheral surface of the heating roller, the contact area between the tension roller and the endless belt becomes comparatively small and the meandering of the endless belt becomes comparatively small.
Incidentally, in recent years, in the fixing device, in order to shorten the warm-up time of the fixing device, the various rollers and the endless belt have become thinner, and the decrease in thermal capacity has progressed. Therefore, in the width direction of the endless belt, temperature unevenness is liable to occur in the endless belt. When the temperature unevenness occurs in the endless belt, unevenness occurs in the degree of fixing a toner image onto a sheet of paper, which may cause the image quality to be deteriorated.
In view of the foregoing, a proposal has been made of a fixing device including a roller that is movable toward and away with respect to the inner peripheral surface of an endless belt and being configured so that, when a temperature in a non-sheet passing area of the endless belt becomes equal to or higher than a predetermined temperature, the roller is brought into contact with the inner peripheral surface of the endless belt so as to release the heat of the endless belt to the roller, and then an excessive temperature rise of the non-sheet passing area of the endless belt is significantly reduced or prevented and reduction in temperature unevenness in the width direction of the endless belt is attained (see Japanese Patent Laid-Open publication No. 2008-275873, for example).
In the fixing device disclosed in Japanese Patent Laid-Open publication No. 2008-275873, since a member in contact with the inner peripheral surface of the endless belt is a roller, the contact area between the roller and the endless belt is small and the heat transfer efficiency is low. Therefore, the reduction effect of the temperature unevenness in the width direction of the endless belt is small. On the other hand, in a case in which the contact area between the member in contact with the inner peripheral surface of the endless belt and the endless belt is made large, although the heat transfer efficiency becomes high and thus the reduction effect of the temperature unevenness becomes high, the meandering of the endless belt may become large since the stabilization of the conveying performance of the endless belt has not been considered in the configuration of conventional fixing devices. In addition, when the endless belt flaps, the contact area between the heating roller and the endless belt changes, the amount of heat that the endless belt receives from the heating roller varies, and thus the temperature of the endless belt may not be stabilized.